For structural materials constituting buildings, rails, bridges and the like, compression or tensile stress is locally applied by wind force, temperature change, the weight of the material or the like. When the stress exceeds a critical value, failure or distortion of the structural materials occurs, often leading to disasters that might cause loss of life, such as destruction of buildings or derailment of trains. For this reason, structural designs and execution have been carried out in consideration of changes in natural environment. In addition, an attempt was made to measure the stress applied to structural materials and control the results, thereby ensuring safety. This method has been partly put into practical use.
The development of a method for magnetically measuring stress applied to structural materials has hitherto been made in the art. In the magnetic measuring method, a magnetic head comprising an exciting head and a detecting head is used to detect magnetic signals, which reflect the magnetic properties of a measured object, to detect a change in the magnetic signals caused by the application of a stress, thereby measuring the stress applied to the object.
Specific examples of prior art techniques for measuring stress include a non-contact stress measuring device, described in Japanese Examined Patent Publication (Kokoku) No. 52-14986, which utilizes such a phenomenon that the coercive force of the measured object varies depending upon the stress. Further, in order to measure the distribution and change of magnetic anisotropy of a measured object using a cross-sensor, Japanese Examined Patent Publication (Kokoku) No. 61-31828 proposes a method for measuring a magnetic anisotropy pattern.
In recent years, attention has been directed to a method using Barkhausen signals attributable to magnetization discontinuities. When a ferromagnetic material is excited, the movement of domain walls within the ferromagnetic material leads to changes in magnetization. The movement of the magnetic walls is discontinuous in a region where precipitates, grain boundaries, and strain are present. Pulsed voltage signals, having a relatively high frequency, corresponding to the discontinuous change are induced in a detecting coil. The pulsed voltage signals are called "Barkhausen signals," which can be applied to the measurement of stress because the intensity of the signal varies depending upon the stress. Regarding this, the following methods and apparatuses have been disclosed. For example, a non-destructive testing method and apparatus for ferromagnetic materials, described in Japanese Unexamined Patent Publication (Kokai) No. 59-112257, can be mentioned as one specific application example of Barkhausen signals. This proposal demonstrates that the stress can be measured with higher accuracy by detecting Barkhausen signals by taking advantage of electromagnetic induction of a detecting coil in a conventional manner and, in addition, detecting Barkhausen signals contained in a simultaneously generated elastic wave by means of a sensor such as a piezoelectric element. Further, Japanese Unexamined Patent Publication (Kokai) No. 60-57247 describes a sensor, for a stress and defect detecting device, characterized by rounding the front end of a ferrite core of a magnetic head. This sensor has been proposed as a magnetic head, which enables stress to be easily measured, because it can detect signals independently of the geometry of the measured object.
The magnetic measuring method had a problem that the measured object is limited to ferromagnetic materials. In this connection, a method has been proposed wherein a ferromagnetic sensor is attached to the surface of a non-magnetic material, thereby enabling, also in the non-magnetic material, the stress to be measured by the magnetic method. Japanese Unexamined Patent Publication (Kokai) No. 61-258161 describes a non-contact magnetic stress and temperature detector. In this detector, two magnetic layers are bonded to the surface of a non-ferromagnetic object to determine a difference in generation time of large Barkhausen signals between the two magnetic layers, and the stress or temperature is measured based on the difference in generation time.
In order to measure the stress with high accuracy using a magnetic measuring method, other factors influencing the magnetic signals than the stress should be separated. Since ferromagnetic materials have a Curie temperature, the magnetic susceptibility, magnetic permeability, coercive force, and magnetic properties of the Barkhausen signals are likely to be influenced by the temperature. For this reason, in order to measure the stress with high accuracy, correction should be made by subtracting the component influenced by the temperature from the magnetic signals, thereby eliminating the influence of the temperature. In the prior art, the stress value has been determined by measuring the temperature of a measured object and correcting the magnetic signals based on the temperature. Since, however, the temperature dependence of the magnetic signals varies for each measuring object and the temperature dependence of the magnetic signals is not always linear, an enormous amount of data for the calibration curves is necessary for correction. Further, the measurement and analysis take a long time, making it difficult to rapidly measure the stress.
When the stress is measured by attaching, as a stress sensor, an element capable of generating Barkhausen signals to a measured object, connecting an exciting head and a detecting head to the stress sensor and measuring the stress based on Barkhausen signals from the detecting head, the Barkhausen signals are influenced by the temperature if the conventional stress sensor is used for the measurement. For this reason, the prior art technique had a problem that an enormous amount of data for calibration with respect to temperature change should be prepared and this resulted in an increased measuring time.